1. Field of the Invention
The invention relates to a color cathode ray tube having a color selection electrode having a tension mask and a frame, which frame includes a plurality of interconnected parts each forming a side of the frame, whereby the tension mask is stretched under tension and connected at attachment points to at least said two parts, the frame having the capability to cause the attachment points to come closer together when the temperature increases.
Such cathode ray tubes are used in for instance television apparatuses and computer monitors.
2. Description of the Related Art
Color cathode ray tubes are usually provided with a color selection electrode, to shadow the electron beam(s) coming from the electron gun(s) mounted in the neck such that each beam excites only one color of luminescent material that is deposited on the inside of a display panel. This color selection is achieved by applying a color selection electrode, for instance a shadow mask inside the tube at a distance from the display panel. This mask employs a large number of apertures, usually arranged in a striped or dotted pattern. Conventional cathode ray tubes have a curved face plate, in most cases resembling either a spherical or a cylindrical surface. Recently more and more color cathode ray tubes have an (almost) flat surface. As a consequence also the color selection electrode has to become more and more flat. However, an (almost) flat shadow mask is very sensitive to doming and microphony problems. It has been known that stretching the shadow mask, i.e., putting it under tension reduces said problems, enabling a flatter shadow mask to be used. Cathode ray tubes having tensed shadow mask suffer, however, from a serious problem. The masks are put under a considerable tension. This tension is at operating temperatures lower than the yield stress of the shadow mask material because otherwise the shadow mask would be plastically deformed. During manufacturing of the cathode ray tube, however, the temperature is raised. Raising the temperature, reduces strongly the yield tension. Plastic deformation of the shadow mask then becomes a real possibility. After plastic deformation the tension in the shadow mask is reduced and in general the position of the apertures in the shadow mask may change. U.S. Pat. No. 5,111,107 discloses a cathode ray tube having a tension shadow mask (in U.S. Pat. No. 5,111,107 a slit-type mask) on a frame. The frame employs two attachment sides on which the mask is attached and two interconnecting sides. The interconnecting sides are provided with bi-metal elements which cause the interconnecting sides to bend inward at increased temperatures causing a reduction of the distance between the connection areas and thereby a reduction of the tension. To avoid plastic deformation the frame thus employs a bimetal element, which reduces the distance between the attachment areas or positions at the opposite sides (which could be seam or several welding points) when the temperature is raised. This reduction in distance releases or at least reduces the tension in the mask to a value (well) below the yield tension at said elvated temperature. Plastic deformation is thereby prevented. Such reduction can also be established by bimetal elements in or on the attachment sides. A design wherein the sides to which the color selection electrode is attached are provided with bimetal element to reduce the distance is for instance known from WO 98/48439. Bimetal element is within the concept to the invention to be understood to include a set of bimetal element or elements in which more than two metal are used. xe2x80x98Metalsxe2x80x99 include metal alloys and metal compounds.
It has, however, been found by inventors that even with a reduction of the distances plastic deformation and/or a relatively poor doming and/or microphony behavior may nevertheless occur in particular at the central area of the mask
It is an object of the invention to improve the doming and/or microphony behavior of a cathode ray tube having a tensed color selection electrode and/or reduce problems caused by plastic deformation of the color selection electrode during manufacturing of the cathode ray tube.
To this end the cathode ray tube according to the invention is characterized in that the attachment parts comprise bimetal elements to reduce, when the temperature is raised, the distance between the attachment areas more at the center of attached sides of the color selection electrode than at the corners of said sides.
The invention is based on the insight that if the distance between the sides is reduced with a substantially equal amount along the attachment sides, the amount of tension released is less at the center of the side than at the corners. As a consequence, if an evenly distributed tension is released by reducing the distance with an equal amount for each side, the amount being such that at the corners the tension is reduced to substantially zero, there still remains a very considerable tension at the center which remaining tension could lead to plastic deformation. By reducing the distance more at the center than at the corners, the remaining tension is reduced.
Thus the risk of plastic deformation is reduced. It is remarked that in some designs (such as for instance the design shown in WO 98/48439) the color selection electrode is unevenly tensioned, i.e. the tension at room temperature is much less in the center than at the corners. In such designs the plastic deformation is less of a problem. Though at high temperatures there is less tension released at the center than at the corners, there was less tension at the center to begin with. However, in such designs the tension at operating temperatures at the center is much less (often less than half) of the maximum tension possible. Compared to such designs, the invention offers the possibility to increase the tension at the center which improves the doming and microphony characteristics of the color selection electrode.
Preferably the ratio of reduction in distance between that at the center and the corners is between 1.3 and 3.
The ratio is chosen to compensate for the effects of a difference in stiffness in the sides. The stiffer the sides are, the smaller the ratio needed, and also the less effect the invention has. For very small ratio (less than 1.3) the positive effects of adding of modifying the bimetal element are relatively small. For high ratio""s the effects of the invention are large, but equally large effects may be more easily obtained by stiffening the sides to first reduce the ratio.
Preferably the frame comprises a further bimetal element in the sides perpendicular to the attachment sides to reduce the distance between the attachment sides as whole.
Within the concept of the invention are embodiments in which bimetal elements on the attachment sides provide both for a reduction in distance of the sides as a whole (which can be seen as a xe2x80x98biasxe2x80x99 reduction of distance), as for a difference in distance between the center and the corners. Preferably however, the attachment sides as a whole are moved closer to each other by a bimetal element in (or on) the sides perpendicular to the attachment sides, and the difference in reduction of distance is achieved by bimetal elements on or in the attachment sides.
Such embodiments are most preferred when the ratio in reduction of distances (as defined above) lies between 1.5 and 2.5. Each bimetal element then achieves an approximately equal effect.
Preferably the cathode ray tube comprises a color selection electrode made of a material having a low coefficient of thermal expansion, such as NiFe alloys.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments hereinafter.